Method for processing video data for a display device

ABSTRACT

The invention is related two a new kind of pre-processing for plasma display panel control. The plasma display technology has the drawback of a reduced grey scale portrayal. This is due to the fact that contrarily to CRTs where luminance is approximately quadratic to the applied cathode voltage, luminance is linear to the number of discharge pulses in PDPs. Therefore, an approximately quadratic degamma function has to be applied to the input video signal components R,G,B before sub-field coding can be done. Truncation to 8-bit video data is required, so that the effect of the degamma function cannot be fully maintained. Especially in the region of small video levels, where the eye sensitivity is high, the grey scale portrayal is poor. According to the invention it is proposed to use a new kind of dithering, adapted to the PDP specialities to improve the grey scale portrayal. These adaptation includes three dithering specialities which can he used singly or in combination. These are: cell-based dithering, i.e. to each colour component R, G, B of a pixel separate dithering numbers are added; object/region-based dithering, i.e. the set of disposable dithering numbers is made dependent on the region/object in the video picture; the set of disposable dithering numbers is made dependent on the video (signal) level.

[0001] The invention relates to a method and apparatus for processingvideo picture data for display on a display device. More specificallythe invention is closely related to a kind of video processing forimproving the picture quality of pictures which are displayed on matrixdisplays like plasma display panels (PDP) or other display devices wherethe pixel values control the generation of a corresponding number ofsmall lighting pulses on the display.

BACKGROUND

[0002] The Plasma technology now makes it possible to achieve flatcolour panel of large size (out of the CRT limitations) and with verylimited depth without any viewing angle constraints.

[0003] Referring to the last generation of European TV, a lot of workhas been made to improve its picture quality. Consequently, a newtechnology like the Plasma one has to provide a picture quality as goodor better than standard TV technology. On one hand, the Plasmatechnology gives the possibility of “unlimited” screen size, ofattractive thickness . . . but on the other hand, it generates new kindsof artefacts which could degrade the picture quality.

[0004] Most of these artefacts are different as for CRT TV pictures andthat makes them more visible since people are used to see the old TVartefacts unconsciously.

[0005] A Plasma Display Panel (PDP) utilizes a matrix array of dischargecells which could only be “ON” or “OFF”. Also unlike a CRT or LCD inwhich grey levels are expressed by analogue control of the lightemission, a PDP controls the grey level by modulating the number oflight pulses per frame (sustain pulses). This time-modulation will beintegrated by the eye over a period corresponding to the eye timeresponse.

[0006] Since the video amplitude determines the number of light pulses,occurring at a given frequency, more amplitude means more light pulsesand thus more “ON” time. For this reason, this kind of modulation isalso known as PWM, pulse width modulation.

[0007] This PWM is responsible for one of the PDP image qualityproblems: the poor grey scale portrayal quality, especially in thedarker regions of the picture. This is due to the fact, that thedisplayed luminance is linear to the number of pulses, but the eyeresponse and its sensitivity to noise is not linear. In darker areas theeye is more sensitive than in brighter areas. This means that eventhough modern PDPs can display e.g. 255 discrete video levels for eachcolour component R,G,B, the quantisation error will be quite noticeablein the darker areas. Further on, the required degamma function in PDPdisplays, increases quantisation noise in video dark areas, resulting ina perceptible lack of resolution.

[0008] There are known some solutions which use a dithering method forreducing the perceptibility of quantisation noise. These solutions arehowever not oriented to the nature of the display and of the displayedvideo. Proposed dithering methods in the literature were mainlydeveloped to improve quality of non-moving black and white images (faxapplication and newspaper photo portrayal). The obtained results aretherefore not optimal if the same dithering algorithms are directlyapplied to PDPs.

INVENTION

[0009] To overcome the drawback of reduced grey scale portrayal, thepresent invention, reports a dithering technique adapted to the specificproblems in PDPs.

[0010] To achieve a better grey scale portrayal, a dithering signal isadded to the video signal, before truncation to the final video greyscale amplitude bit resolution. As mentioned before, dithering per se isa well-known technique from the technical literature, used to reduce theeffects of quantisation noise due to a reduced number of displayedresolution bits. With dithering, some artificial levels are addedin-between the existing video levels. This improves the grey scaleportrayal, but on the other hand adds high frequency, low amplitudedithering noise which is perceptible to the human viewer only at a smallviewing distance.

[0011] The solution according to the invention makes an adaptation ofthe dithering signal to the PDP specialities in order to achieve anoptimised grey-scale portrayal and a minimised dithering noise at thesame time. There are three concrete techniques which can be used singlyor in combination for the optimisation. These are:

[0012] Cell-based dithering: adaptation to the cell structure of theplasma display.

[0013] Object-based dithering: adaptation to the structure of thedisplayed video picture.

[0014] Amplitude-based dithering: adaptation to the amplitude level ofthe pixels or pixel regions in the displayed video picture.

[0015] Cell-based dithering consists in adding a dithering signal thatis defined for every plasma cell (there are 3 plasma cells R,G,B foreach pixel) and not for every pixel. This makes the dithering noisefiner and less noticeable to the human viewer.

[0016] Object-based dithering means enabling addition of a ditheringsignal only for certain picture content objects, or to adapt the set ofdisposable dithering numbers to the bit resolution of the displayedobjects. In other words, the bit resolution for the dithering numbers ismade adaptive to the bit resolution of the displayed objects. Twoexamples will help to clarify this idea:

[0017] 1. OSD (On-Screen Display) is mostly generated with 4-bits ofresolution per colour component R,G,B. This means that the display greyscale resolution (8 bit for each colour component R,G,B) is more thanenough to correctly portray this kind of OSD, and therefore adding adithering signal would only add dithering noise, without bringing anoticeable benefit.

[0018] 2. If a PC graphic card is connected to the plasma display, forinstance in 256-color mode, it is also useless to add a ditheringsignal. The bit resolution for each colour component R,G,B is also verylow in this mode. Use of a dithering technique would not improve thegrey scale portrayal. It is likely that the graphics card would add inseries an own dithering signal to compensate for the reduced number ofcolours.

[0019] Amplitude-based dithering means that the set of disposabledithering numbers is made a function of the amplitude of the videosignal components. Also here, in other words, this could be expressedthat the bit resolution for the dithering numbers is made adaptive tothe video signal component amplitude. Contrary to the smaller (darker)video values, large values of video do not loose bit resolution with theapplication of the quadratic degamma function. Therefore, the number ofdithering bits can be reduced as a function of the amplitude.

[0020] Further advantageous embodiments are apparent from the dependentclaims.

DRAWINGS

[0021] Exemplary embodiments of the invention are illustrated in thedrawings and are explained in more detail in the following description.

[0022]FIG. 1 shows an illustration for the plasma cell activation withsmall pulses in sub-fields;

[0023]FIG. 2 shows an illustration for pixel-based and cell-baseddithering;

[0024]FIG. 3 shows an illustration of a 3-dimensional cell-baseddithering pattern;

[0025]FIG. 4 shows a block diagram of a circuit implementation of theinvention in a PDP.

EXEMPLARY EMBODIMENTS

[0026] In FIG. 1, the general concept of light generation in plasmadisplay panels is illustrated. As mentioned before, a plasma cell canonly be switched on or off. Therefore, the light generation is beingdone in small pulses where a plasma cell is switched on. The differentcolours are produced by modulating the number of small pulses per frameperiod. To do this a frame period is subdivided in so called sub-fieldsSF. Each sub-field SF has assigned a specific weight which determineshow many light pulse are produced in this sub-field SF. Light generationis controlled by sub-field code words. A sub-field code word is a binarynumber which controls sub-field activation and inactivation. Each bitbeing set to 1 activates the corresponding sub-field SF. Each bit beingset to 0 inactivates the corresponding sub-field SF. In an activatedsub-field SF the assigned number of light pulses will be generated. Inan inactivated sub-field there will be no light generation. A typicalsub-field organisation with 12 sub-fields SF is shown in FIG. 1. Thesub-field weights are listed at the top of the figure.

[0027] The frame period is illustrated slightly longer than all thesub-field periods together. This has the reason that for non-standardvideo sources the video line may be subject of jittering and to makesure that all sub-fields SF fits into the jittering video line, thetotal amount of time for all sub-fields SF is slightly shorter than astandard video line.

[0028] For clarification, a definition of the term sub-field is givenhere: A sub-field is a period of time in which successively thefollowing is being done with a cell:

[0029] 1. There is a writing/addressing period in which the cell iseither brought to an excited state with a high voltage or with lowervoltage to a neutral state.

[0030] 2. There is a sustain period in which a gas discharge is madewith short voltage pulses which lead to corresponding short lightingpulses. Of course only the cells previously excited will producelighting pulses. There will not be a gas discharge in the cells inneutral state.

[0031] 3. There is an erasing period in which the charge of the cells isquenched.

[0032] As mentioned before, plasma uses PWM (pulse width modulation) togenerate the different shades of grey. Contrarily to CRTs whereluminance is approximately quadratic to the applied cathode voltage,Luminance is linear to the number of discharge pulses in PDPs.Therefore, an approximately quadratic degamma function has to be appliedto the input video signal components R,G,B before the PWM.

[0033] The effect of this degamma function on the input video data isshown in the following table, where a quadratic degamma function isapplied (calculated with 16-bit resolution). After applying thequadratic degamma function to the input video data, in the next columnthe effect of this degamma function is depicted. The numbers in thiscolumn were achieved after dividing the quadratic numbers in theprevious column by 256 and truncation. By doing this it is assured thatthe output video range and the input video range is identical. 11 Bit 8Bit Input 16 Bit 8 Bit Out- 11 Bit 8 Bit Input 16 Bit De- 8 Bit OutputDegamma Video Degamma put Video Degamma Video Data gamma Data Video DataData Data Data Data Data (X) (X**2) (X**2/255) (X**2)/32 (X) (X**2)(X**2/255) (X**2)/32 0 0 0 0 128 16384 64 512 1 1 0 0 129 16641 65 520 24 0 0 130 16900 66 528 3 9 0 0 131 17161 67 536 4 16 0 0 132 17424 68544 5 25 0 0 133 17689 69 552 6 36 0 1 134 17956 70 561 7 49 0 1 13518225 71 569 8 64 0 2 136 18496 72 578 9 81 0 2 137 18769 73 586 10 1000 3 138 19044 74 595 11 121 0 3 139 19321 75 603 12 144 0 4 140 19600 76612 13 169 0 5 141 19881 77 621 14 196 0 6 142 20164 79 630 15 225 0 7143 20449 80 639 16 256 1 8 144 20736 81 648 17 289 1 9 145 21025 82 65718 324 1 10 146 21316 83 666 19 361 1 11 147 21609 84 675 20 400 1 12148 21904 85 684 21 441 1 13 149 22201 87 693 22 484 1 15 150 22500 88703 23 529 2 16 151 22801 89 712 24 576 2 18 152 23104 90 722 25 625 219 153 23409 91 731 26 676 2 21 154 23716 93 741 27 729 2 22 155 2402594 750 28 768 3 24 156 24336 95 760 29 841 3 26 157 24649 96 770 30 9003 28 158 24964 97 780 31 961 3 30 159 25281 99 790 32 1024 4 32 16025600 100 800 33 1089 4 34 161 25921 101 810 34 1156 4 36 162 26244 102820 35 1225 4 38 163 26569 104 830 36 1296 5 40 164 26896 105 840 371369 5 42 165 27225 106 850 38 1444 5 45 166 27556 108 861 39 1521 5 47167 27889 109 871 40 1600 6 50 168 28224 110 882 41 1681 6 52 169 28561112 892 42 1764 6 55 170 28900 113 903 43 1849 7 57 171 29241 114 913 441936 7 60 172 29584 116 924 45 2025 7 63 173 29929 117 935 46 2116 8 66174 30276 118 946 47 2209 8 69 175 30625 120 957 48 2304 9 72 176 30976121 968 49 2401 9 75 177 31329 122 979 50 2500 9 78 178 31684 124 990 512601 10 81 179 32041 125 1001 52 2704 10 84 180 32400 127 1012 53 280911 87 181 32761 128 1023 54 2916 11 91 182 33124 129 1035 55 3025 11 94183 33489 131 1046 56 3136 12 98 184 33856 132 1058 57 3249 12 101 18534225 134 1069 58 3364 13 105 186 34596 135 1081 59 3481 13 108 18734969 137 1092 60 3600 14 112 188 35344 138 1104 61 3721 14 116 18935721 140 1116 62 3844 15 120 190 36100 141 1128 63 3969 15 124 19136481 143 1140 64 4096 16 128 192 36864 144 1152 65 4225 16 132 19337249 146 1164 66 4356 17 136 194 37636 147 1176 67 4489 17 140 19538025 149 1188 68 4624 18 144 196 38416 150 1200 69 4761 18 148 19738809 152 1212 70 4900 19 153 198 39204 153 1225 71 5041 19 157 19939601 155 1237 72 5184 20 162 200 40000 156 1250 73 5329 20 166 20140401 158 1262 74 5476 21 171 202 40804 160 1275 75 5625 22 175 20341209 161 1287 76 5776 22 180 204 41616 163 1300 77 5929 22 185 20542025 164 1313 78 6084 23 190 206 42436 166 1326 79 6241 24 195 20742849 168 1339 80 6400 25 200 208 43264 169 1352 81 6561 25 205 20943681 171 1365 82 6724 26 210 210 44100 172 1378 83 6889 27 215 21144512 174 1391 84 7056 27 220 212 44944 176 1404 85 7225 28 225 21345369 177 1417 86 7396 29 231 214 45796 179 1431 87 7569 29 236 21546225 181 1444 88 7744 30 242 216 46656 182 1458 89 7921 31 247 21747089 184 1471 90 8100 31 253 218 47524 186 1485 91 8281 32 258 21947961 188 1498 92 8464 33 264 220 48400 189 1512 93 8649 33 270 22148841 191 1526 94 8836 34 276 222 49284 193 1540 95 9025 35 282 22349729 195 1554 96 9216 36 288 224 50176 196 1568 97 9409 36 294 22550625 198 1582 98 9604 37 300 226 51076 200 1596 99 9801 38 306 22751529 202 1610 100 10000 39 312 228 51984 203 1624 101 10201 40 318 22952441 205 1638 102 10404 40 325 230 52900 207 1653 103 10609 41 331 23153361 209 1667 104 10816 42 338 232 53824 211 1682 105 11025 43 344 23354289 212 1696 106 11236 44 351 234 54756 214 1711 107 11449 44 357 23555225 216 1725 108 11664 45 364 236 55696 218 1740 109 11881 46 371 23756169 220 1755 110 12100 47 378 238 56644 222 1770 111 12321 48 385 23957121 224 1785 112 12544 49 392 240 57600 225 1800 113 12769 50 399 24158081 227 1815 114 12996 50 406 242 58564 229 1830 115 13225 51 413 24359049 231 1845 116 13456 52 420 244 59536 233 1860 117 13689 53 427 24560025 235 1875 118 13924 54 435 246 60516 237 1891 119 14161 55 442 24761009 239 1906 120 14400 56 450 248 61504 241 1922 121 14641 57 457 24962001 243 1937 122 14884 58 465 250 62500 245 1953 123 15129 59 472 25163001 247 1968 124 15376 60 480 252 63504 249 1984 125 15625 61 488 25364009 251 2000 126 15876 62 496 254 64516 253 2016 127 16129 63 504 25565025 255 2032

[0034] As it can be seen from the values in the columns headed 8 bitoutput video data, for smaller input values, many input levels aremapped to the same output level. This is due to division by 255 andtruncation. In other words, for darker areas, the quantisation step ishigher than for the higher areas which corresponds to non-linearquantisation. In particular the values smaller than 16 are all mapped to0 (this corresponds to four bit video data resolution which isunacceptable for video signal processing).

[0035] Dithering is a known technique for avoiding to loose amplituderesolution bits due to truncation This technique only works if therequired resolution is available before the truncation step. But this isthe case in the present application, because the video data afterdegamma operation has 16 bit resolution and in the corresponding columnsthere are no two identical values. Dithering can in principle bring backas many bits as those lost by truncation. However, the dithering noisefrequency decreases, and therefore becomes more noticeable, with thenumber of dithering bits.

[0036] 1 bit-dithering corresponds to multiply the number of availableoutput levels by 2, 2 bit-dithering corresponds to multiply the numberof available output levels by 4 and 3 bit-dithering corresponds tomultiply the number of available output levels by 8.

[0037] Looking at the table above, in particular to the input valuesless than 16 reveals that at minimum 3 bit-dithering is required toreproduce the 256 video levels more correctly with the required greyscale portrayal of a ‘CRT’ display device.

[0038] In the table above the columns headed 11 Bit Degamma Data containthe output data from the degamma unit. These values are derived from thevalues in the columns headed 16 Bit Degamma data by dividing them by 32or better by truncation of 5 bits. How these values are used in thedithering process will be explained later on.

[0039] Next, the cell-based dithering will be explained in detail.

[0040] With cell-based dithering a dithering number is added to everypanel cell in contrast to every panel pixel as usually done. A panelpixel is composed of three cells: red, green and blue cell. Thecell-based dithering has the advantage of rendering the dithering noisefiner and thus making it less noticeable to the human viewer.

[0041] Because the dithering pattern is defined cell-wise, it is notpossible to use techniques like error-diffusion, in order to avoidcolouring of the picture when one cell would diffuse in the contiguouscell of a different colour. This is not a big disadvantage, because ithas been observed sometimes an undesirable low frequency movinginterference, between the diffusion of the truncation error and a movingpattern belonging to the video signal. Error diffusion works best incase of static pictures.

[0042] Instead of using error diffusion, a static 3-dimensionaldithering pattern is proposed according to this invention.

[0043]FIG. 3 shows one example for such a pattern. 3-bit-dithering isused in this example. This means that the dithering numbers have valuesfrom 0 to 7. The static 3-dimensional dithering pattern is defined for acube of 4*4*4 cells (4-lines with 4 cells each, repeatedly taken from 4frames). It is noted that this embodiment is only an example and thatthe number of dithering bits as well as the size and type of ditheringpattern can be subject of modification in other embodiments of theinvention.

[0044] The use of a 3 bit-dithering requires that the degamma operationis performed with 3 bits more than final resolution. The finalresolution is given to be 8 bit resolution. The sub-field coding rangeis therefore from 0 to 255. Then the output range of the degammaoperation should be from 0 to 2040. It is noted that the maximumdithering number with 3 bit dithering is 7. If this number is added to2040, the result is 2047 which is the highest possible 11 bit binarynumber % 11111111111. A slightly lower value than 2040. e.g. 2032 couldalso be used. This has the advantage that the corresponding values cansimply be derived from the 16 bit degamma data by truncating the 5 leastsignificant bits.

[0045] Some other examples: if sub-field coding range would be from 0 to175, output range of degamma operation should be from 0 to 1400; andfinally if coding range is from 0 to 127, output range should be from 0to 1016. For every panel cell and for every frame, the correspondingdither pattern value is added to the output of the degamma function, andconsequently truncated to the final number of bits.

[0046] The 3-bit dither pattern shown in FIG. 3 is static. This meansthat it is repeatedly used for the whole panel. From FIG. 3 it can beseen that the dither pattern is repeated in horizontal direction of thepanel. However, it also repeats in vertical direction and in timedirection accordingly.

[0047] It is noted that the proposed pattern, when integrated over time,always gives the same value for all panel cells. If this were not thecase, under some circumstances, some cells could acquire an amplitudeoffset compared to other cells which would correspond to an undesirablefixed spurious static pattern.

[0048] Next, the principle of object-based dithering according to theinvention is explained in greater detail. Object-based ditheringcorresponds to modify the number of dithering bits as a function of thedisplayed object. For this purpose different masking bit patterns aredefined which serve as a selector for the dithering bit resolution.E.g., if the object-based dithering is used in combination with thecell-based dithering, the implementation of different dithering bitresolutions can be done as follows.

[0049] The dithering pattern as shown in FIG. 3 remains unchanged. I.e.,the dithering numbers have the 3 bit resolution as before at thebeginning of the dithering process. This is the highest possible bitresolution in this example. For implementing the 4 different bitresolutions 3-bit, 2-bit, 1-bit and 0-bit, 4 different masking valuesare defined. These are:

[0050] 3-bit dithering->masko=%111=7H

[0051] 2-bit dithering->masko=%110=6H

[0052] 1-bit dithering->masko=%100=4H

[0053] 0-bit dithering->masko=%000=OH

[0054] These masking bit patterns are applied to the high resolutiondithering numbers by Boolean operation. This can best be explained withsome examples. In the examples below the Boolean operation is thelogical AND operation. 3-Bit Dithering Dithering Number Masking BitPattern Result %111 %111 %111 %110 %111 %110 %101 %111 %101 %100 %111%100 %011 %111 %011 %010 %111 %010 %001 %111 %001 %000 %111 %000

[0055] 2-Bit Dithering Dithering Number Masking Bit Pattern Result %111%110 %110 %110 %110 %110 %101 %110 %100 %100 %110 %100 %011 %110 %010%010 %110 %010 %001 %110 %000 %000 %110 %000

[0056] 1-Bit Dithering Dithering Number Masking Bit Pattern Result %111%100 %100 %110 %100 %100 %101 %100 %100 %100 %100 %100

[0057] Dithering Number Masking Bit Pattern Result %011 %100 %000 %010%100 %000 %001 %100 %000 %000 %100 %000

[0058] 0-Bit Dithering Dithering Number Masking Bit Pattern Result %111%000 %000 %110 %000 %000 %101 %000 %000 %100 %000 %000 %011 %000 %000%010 %000 %000 %001 %000 %000 %000 %000 %000

[0059] From the table for 3-bit dithering it is clear that the appliedmasking bit pattern has no effect on the dithering numbers. They remainunchanged and therefore, 3-bit dithering is preserved as wanted.

[0060] From the table for 2-bit dithering it is clear that the appliedmasking bit pattern converts the 3-bit dithering numbers into 2-bitdithering numbers. There result only 4 different output values whichcorresponds to 2-bit dithering as wanted.

[0061] From the table for 1-bit dithering it is clear that the appliedmasking bit pattern converts the 3-bit dithering numbers into 2-bitdithering numbers. There result only 2 different output values whichcorresponds to 1-bit dithering as wanted.

[0062] From the table for 0-bit dithering it is clear that the appliedmasking bit pattern converts the 3-bit dithering numbers into 0-bitdithering numbers. Every input dithering number is converted to 0 whichcorresponds to 0-bit dithering as wanted.

[0063] The dithering bit resolution selection with masking bit patternshas the advantage that there need not be different tables for ditheringpatterns and different algorithms. So that the presented solution isvery efficient.

[0064] In a practical application OSD insets are coded with 0-bitdithering while the video picture is coded with 3-bit dithering. If theplasma display panel is used as a monitor for computers, window bordersand icons, as well as documents might be displayed with 0-bit dithering,while wall-papers and windows with motion pictures (video scenes), e.g.AVI-files or MPG-files might have 1-bit, 2-bit or 3-bit ditheringenabled.

[0065] If a video picture has been coded according to the MPEG-4standard the object/region-based dithering can benefit from this coding.The MPEG-4 standard provides the tools for video object coding. Thismeans that the different objects in a video scene are codedindependently. In a further embodiment of the invention the number ofdithering bits for the cells of an object in a picture are adapted tothe kind and to the bit-resolution of the objects belonging to a givenMPEG-4 sequence. For instance very often the background is darker thanthe rest of the picture and has low contrast. In this region theapplication of 3-bit dithering is therefore used. The foreground is veryoften brighter and mostly more rich in contrast. In this region 1 bitdithering is therefore more appropriate.

[0066] Of course, object-based dithering requires some kind ofinformation from the video source regarding video objects. This requiresa picture content analysis which can be very complicated to implement.If in a low cost application this picture content analysisimplementation is considered to be too expensive, then a low costimplementation of object-based dithering can be the restriction toswitching off dithering in case of On-Screen-Display insets andswitching on dithering for the rest of the picture.

[0067] Next, the principle of amplitude-based dithering according to theinvention is explained in greater detail. Amplitude-based ditheringcorresponds to modify the number of dithering bits as a function of thevideo component signal amplitude. This can be done in similar fashionlike for the object-based dithering. There are also defined some maskingbit patterns for the different amplitude ranges which are used to selecta corresponding dithering bit resolution by Boolean operation with thedithering numbers.

[0068] In video technology the video signal component value range isusually from 0 to 255 (8 bit words). This range is subdivided in e.g. 4sections. The ranges and the assigned corresponding masking bit patternsare shown below:

[0069] For (0<=X<32), maska=%111=7H,

[0070] for (32<=X<64), maska=%110=6H,

[0071] for (64<=X<128), maska=%100=4H,

[0072] for (128<=X<=255), maska=%000=0H,

[0073] where X is the amplitude of the input video component R,G,B.

[0074] According to this embodiment of the invention in the ditheringcircuit section the input video signal components will be classifiedwith respect to the amplitude range. The dithering number from thedithering pattern is taken in 3-bit resolution and the logical ANDoperation is performed with the corresponding masking bit pattern. Theresulting value is added to the video signal component data. This isdone separately for each cell. The same principle is used forobject-based dithering.

[0075] Next, it is explained in greater detail how the three differentdithering techniques, cell-, amplitude- and object-based dithering canbe combined for an optimisation.

[0076] Taking in consideration the above mentioned example with 3-bitdithering numbers, a combined solution can be described with thefollowing formulae:

Rout=trunc[degamma[Rin]+(rdither[x,y,z] AND maska[Rin,x,y,z] ANDmasko[x,y,z])]

Gout=trunc[degamma[Gin]+(gdither[x,y,z] AND maska[Gin,x,y,z] ANDmasko[x,y,z])]

Bout=trunc[degamma[Bin]+(bdither[x,y,z] AND maska[Bin,x,y,z] ANDmasko[x,y,z])]

[0077] where

[0078] Rin denotes the video level of the red input video signalcomponent R,

[0079] Gin denotes the video level of the green input video signalcomponent G,

[0080] Bin denotes the video level of the blue input video signalcomponent B,

[0081] degamma[ ] denotes the degamma function with 11 bit resolution,

[0082] maska[ ] denotes the amplitude-based masking value,

[0083] masko[ ] denotes the object-based masking value,

[0084] rdither[ ] denotes the cell based dithering number for the redcells according to the used dithering pattern,

[0085] gdither[ ] denotes the cell based dithering number for the greencells according to the dithering pattern,

[0086] bdither[ ] denotes the cell based dithering number for the bluecells according to the dithering pattern,

[0087] x denotes the panel pixel number,

[0088] y denotes the panel line number,

[0089] z denotes the frame number and

[0090] trunc [ ] denotes truncation to 8 bit resolution, i.e. truncationof the 3 least significant bits.

[0091] The expressions:

[0092] (rdither [x,y,z] AND maska [Rin,x,y,z] AND masko [x,y,z])],

[0093] (gdither [x,y,z] AND maska [Gin,x,y,z] AND masko [x,y,z])],

[0094] (bdither [x,y,z] AND maska [Bin,x,y,z] AND masko [x,y,z])]

[0095] therefore denote a resulting dithering number after combinationwith the masking bit patterns from object- and amplitude-baseddithering.

[0096] The results of this calculations is illustrated in the followingtables below. The results are only shown exemplarily for three inputvalues 8, 21, 118. This is because the full table cannot be easilydisplayed on paper. The effect of dithering is however obvious alreadyfrom the tables below. The first table concerns the example of 3-bitdithering. It is evident that for the input value 8 due to dithering theoutput value is changed from 0 to 1 in two cases compared to theembodiment without dithering. For the input value 21 the output value ischanged from 1 to 2 in five cases compared to the case withoutdithering. For the input value 118 the output value is changed from 54to 55 in three cases. Of course, the effect of dithering is becomingsmaller as the input value increases because the ratio between ditheringvalue to input value decreases.

[0097] Maska=masko=%111=3-bit dithering 8 Bit Input 16 Bit De- 8 Bit De-11 Bit De- Dithering 8 Bit Output Video Data gamma Data gamma Data gammaData Number Video Data 8 64 0 2 7 1 6 1 5 0 4 0 3 0 2 0 1 0 0 0 21 441 113 7 2 6 2 5 2 4 2 3 2 2 1 1 1 0 1 118 13924 54 435 7 55 6 55 5 55 4 543 54 2 54 1 54 0 54

[0098] The next table lists the calculation results for 2-bit dithering.Here, the effect of dithering is of course getting smaller, as smallerdithering numbers are added. However, a difference is present only forthe input value 18 where the output value is changed in only four casesand for the input value 118, where the output value is changed from 54to 55 in only two cases. Maska = masko = %110 = 2-bit dithering 8 BitInput 16 Bit De- 8 Bit De- 11 Bit De- Dithering 8 Bit Output Video Datagamma Data gamma Data gamma Data Number Video Data 8 64 0 2 7 1 6 1 5 04 0 3 0 2 0 1 0 0 0 21 441 1 13 7 2 6 2 5 2 4 2 3 1 2 1 1 1 0 1 11813924 54 435 7 55 6 55 5 54 4 54 3 54 2 54 1 54 0 54

[0099] The next table lists the calculation results for 1-bit dithering.Here, the effect of dithering has vanished for the input vales 8 and 118but for the input value 21 there is still the effect that the outputvalues have been changed from 1 to 2 in four cases. Of course there areother input values, like 12, where the effect is maintained. Maska =masko = %100 = 1-bit dithering 8 Bit Input 16 Bit De- 8 Bit De- 11 BitDe- Dithering 8 Bit Output Video Data gamma Data gamma Data gamma DataNumber Video Data 8 64 0 2 7 1 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 21 441 113 7 2 6 2 5 2 4 2 3 1 2 1 1 1 0 1 118 13924 54 435 7 54 6 54 5 54 4 543 54 2 54 1 54 0 54

[0100] In FIG. 4 a circuit implementation of the invention isillustrated. Input R,G,B video data is forwarded to degamma unit 10 anda dither evaluation unit 12. The degamma unit 10 performs the 11-bitdegamma function and delivers 11 bit video data R,G,B at the output. Thedither evaluation unit 12 computes the dithering numbers: DR for red, DGfor green and DB for blue. To do that it requires the sync signals H andV to determine which pixel is currently processed and which line andframe number is valid. These information is used for addressing a lookuptable in which the dithering pattern is stored. The R, G and Bcomponents are used in this unit for evaluating the amplitude maskingvalues maska. The masking value MO, which is the object-based maskingvalue for the current pixel, is delivered by a unit in the video source,like MPEG4 decoder. This unit is not shown. In the case that no suchunit is available, the signal MO can be replaced by the fast blankingsignal of an external OSD insertion circuit. Unit 12 also performs theBoolean operations according to above discussed formulae. In calculationunit 11 the resulting dithering numbers and the degamma output valuesare added and the 3 least significant bits of the result are truncatedso that the final output values Rout, Gout and Bout are achieved. Thesevalues are forwarded to a sub-field coding unit 13 which performssub-field coding under control of control unit 16. The sub-field codewords are stored in memory unit 14. Reading and writing from and to thismemory unit is also controlled by the external control unit 16. Forplasma display panel addressing, the sub-field code words are read outof the memory device and all the code words for one line a collected inorder to create a single very long code word which can be used for theline wise PDP addressing. This is carried out in the serial to parallelconversion unit 15. The control unit 16 generates all scan and sustainpulses for PDP control. It receives horizontal and verticalsynchronising signals for reference timing.

[0101] The invention can be used in particular in PDPs. Plasma displaysare currently used in consumer electronics, e.g. for TV sets, and alsoas a monitor for computers. However, use of the invention is alsoappropriate for matrix displays where the light emission is alsocontrolled with small pulse in sub-fields, i.e. where the PWM principleis used for controlling light emission.

1. Method for processing video picture data for display on a displaydevice having a plurality of luminous elements corresponding to thecolour components of pixels of a video picture, wherein a ditheringmethod is applied to the video data to refine the grey-scale portrayalin the video pictures, characterized in that, the dithering methodincludes one or more of the following specialities singly or incombination: the dithering is made cell-based, i.e. to each colourcomponent R, G, B of a pixel separate dithering numbers are added; theset of disposable dithering numbers is made dependent on theregion/object in the video picture; the set of disposable ditheringnumbers is made dependent on the video (signal) level.
 2. Methodaccording to claim 1, wherein for the cell-based dithering the videopicture is divided into a number of sections and a static 3-dimensionaldithering pattern is defined which is used repeatedly in a videosequence, wherein a first dimension corresponds to a video line number,a second dimension corresponds to a number of pixels within a video linesection and a third dimension corresponds to a number of video frames.3. Method according to claim 2, wherein the static 3-dimensionaldithering pattern is defined for a section of 4 lines with 4 pixels eachfor a number of 4 consecutive frames with a bit-resolution for thedithering numbers of 3 bits.
 4. Method according to one of the previousclaims, wherein for the region/object-based dithering, the informationabout different video objects/region is taken over from an MPEG4 datastream.
 5. Method according to one of the previous claims, wherein toeach of the specific sets of disposable dithering numbers for thedithering process, a corresponding masking bit pattern is assigned whichdetermines by Boolean operation which of the bits of a high resolutiondithering number are to be taken for the resulting final ditheringnumber.
 6. Method according to one of the previous claims, wherein forthe video level-based dithering, the full video level range issubdivided in a number of sections and to each section a correspondingmasking bit pattern is assigned which determines by Boolean operationwhich of the bits of a high resolution dithering number are to be takenfor the resulting final dithering number.
 7. Method according to claim6, wherein the full video level range from 0 to 255 is subdivided in 4sections, in particular 0 to 31, 32 to 63, 64 to 127, and 128 to 255 andcorrespondingly the following bit resolutions are used for the ranges,3-bit, 2-bit, 1-bit, 0-bit and wherein the bit resolution decreases asthe video level range increases.
 8. Method according to one of theprevious claims, wherein for the combined use of all the ditheringspecialities the following formulae are applied:Rout=trunc[degamma[Rin]+(rdither[x,y,z] AND maska[Rin,x,y,z] ANDmasko[x,y,z])]Gout=trunc[degamma[Gin]+(gdither[x,y,z] ANDmaska[Gin,x,y,z] ANDmasko[x,y,z])]Bout=trunc[degamma[Bin]+(bdither[x,y,z] ANDmaska[Bin,x,y,z] AND masko[x,y,z])], where Rin denotes the video levelof the red input video signal component R, Gin denotes the video levelof the green input video signal component G, Bin denotes the video levelof the blue input video signal component B, degamma[ ] denotes thedegamma function with specific bit resolution, in particular 11-bitresolution, maska[ ] denotes the amplitude-based masking value, masko[ ]denotes the object-based masking value, rdither[ ] denotes thecell-based dithering number for the red cells according to the useddithering pattern, gdither[ ] denotes the cell-based dithering numberfor the green cells according to the used dithering pattern, bdither[ ]denotes the cell-based dithering number for the blue cells according tothe used dithering pattern, x denotes the panel pixel number, y denotesthe panel line number, z denotes the frame number, and trunc[ ] denotestruncation to a specific bit resolution, in particular 8-bit resolution.9.) Use of the method according to one of the previous claims, for thevideo signal processing in a Plasma display device. 10.) Apparatus forprocessing video pictures for display on a display device having aplurality of luminous elements corresponding to the colour components ofpixels of a video picture, said apparatus comprising a dithering unitwhich calculates dithering numbers which are added to video picture datato refine the grey-scale portrayal in the video pictures, characterizedin that, the dithering unit calculates dithering numbers according toone or more of the following specialities singly or in combination: thedithering number calculation is made cell-based, i.e. to each colourcomponent R, G, B of a pixel separate dithering numbers are added; theset of disposable dithering numbers is made dependent on theregion/object in the video picture; the set of disposable ditheringnumbers is made dependent on the video (signal) level.